Secondary electron emitter



Patented Dec. 11, 1945 SECONDARY ELECTRON EMITTEB:

Warren R. Ferris. Kingston, N. J., assignor. to Radio Corporation of America, a corporation of Delaware Application January 27, 1943, Serial No. 473,663 3 Claims. (01. 250-174) My invention relates to electrodes which are capable of emitting secondary electrons in a ratio greater than unity upon impact by primary electrons at moderate voltages, and more particularly to such electrodes suitable for use as secondary electron emitters in electron discharge devices which have thermionic cathodes and in which the secondary electron emitter is sub- Jected to primary electron bombardment of such intensity that the secondary electron emitter may be heated up to a red heat.

In general, a satisfactory secondary electron emitter should be able to operate at 300 C. with a primary current density of about 5 milliamperes per square centimeter in a good vacuum for a reasonable life, such as 400 hours or more, without too great a change in gain where gain is the ratio of secondary electrons to primary or impacting electrons.

Secondary electron emitters of the caesium oxide silver type in which caesium is condensed on an oxidized silver surface have a rather high ratio of emission of secondary electrons to primary electrons at moderate voltages, but deteriorate rapidly if the emitter is operated at a. temperature as high as about 200 C. For this reason, such secondary electron emitters are not desirable for electron discharge devices in which loy containing a substantial percentage of chromium, the balance preferably consisting of one or more metals of the iron group, which includes nickel, iron and cobalt. These metals may be pure or of commercial grades. Commercial nickel contains some cobalt, which acts like nickel so far as my invention is concerned. I prefer the nickel base alloy known commercially as Nichrome and consisting of about 80% nickel and chromium. Other varieties of commercial Nichrome, containing from 60% to 75% nickel, from 11% to 16% chromium, and 12% to 24% iron, and other chromium containing alduring normal operation the secondary electron uniform gain or efllciency for a reasonable life and over a rather wide temperature range. Another obiectis to provide a method of makin such a secondary electron emitter. Still another object is to provide a secondary electron emit-" ter which is rugged and resistant to high temperature, and which during a reasonable life remains eflicient and operative over a wide range of temperature up to a temperature above that at which a caesiated oxidized silver secondary electron emitter becomes inoperative.

In accordance with my invention the secondary electron emitter consists of a conductive base or foundation having on its surface a film containing a substantial proportion of chromic oxide and activated by an alkali metal such as caesium, or by an alkaline earth metal, condensed on the chromic oxide from the vapor Phase. The foundation of the electrode is preferably an alloys. such as chrome iron may also be used. It is not essential that the chromic oxide containing film on the foundation be obtained from an' alloy having metal of the iron group as a base, since a chromic oxide containing film may also be produced on a base having a chromium surface, such as chromium plated iron. My observations indicate that th chromic oxide containing film should preferably be so thin that it shows interference colors, indicating a .thickness of about a micron. or from 10- centimeters to 10" centimeters. When the film is so thick that it shows the typical greenish color of chromic oxide, the results are not as good as when the thickness of the film is within the range where the interference colors appear. This chromic oxide containing film is activated in the usual way by condensing upon it from the vapor phase an activating metal, such as an alkali metal or an alkaline earth metal, and driving off the excess activating metal to leave a very thin and usually submicroscopic surface coating of the activating metal. I prefer to use as an activating metal an alkali metal such as caesium or rubidium, both of which have given good results.

The preferred thin chromic oxide-containing film showing interference colors may be formed by lightly oxidizing the chromium alloy foundation by oxygen at reduced pressure. The thick green chromic oxide containing film on the surface of the foundation may be formed by heating the chromium alloy foundation in an atmosphere of moist hydrogen. As the chromic oxide is very stable, it is probable that most of the thick greenish film consists of chromic oxide, as the conditions are not favorable for the formation of oxides of the other constituents of the al- 10y. The proportion of chromic oxide in the film does not seem to be critical as long as itis present in substantial proportions.

It is believed that chromic oxide is a necessary constituent or the film on the foundation since both the chemical and physical stability of chromic oxide at high temperatures is known to be very great. It may be that the caesium or other ide is obtained from Nichrome or from pure chromium, chrome iron, or other chromium-containing alloys having metal of the iron group as a interference colors.

base. As long as the oxide film on the foundation contains a substantial quantity of chromic oxide as a constituent of a film having a thickness such that it shows interference colors, the advantages of my invention may be obtained by activating the film with a suitable activating metal, such as the alkali metals, preferably caesium or rubidium, condensed on the film from vapor of the activating metal.

The secondary electron emitter constructed in accordance with my invention is particularly useful in an electron discharge device having a thermionic cathode for producing a primary electron stream of such intensity and velocity that the secondary emitter is subjected to an electron bombardment which raises its temperature above that at which a caesiated oxidized silver surface becomes inoperative.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 is a transverse section of one form of electron discharge device utilizing a secondary electron emitter made according to my invention; and Fig. 2 is a diagrammatic section on a greatly enlarged scale of a secondary electron emitter embodying my invention.

In Fig. 1 of the accompanying drawing I show, merely for purposes of illustration, one form of tube in which good results have ben obtained with secondary electron emitters constructed in accordance with my invention. This particular type of tube, shown and described more in detail in U. S. patent to Wagner, 2,272,232, February 10, 1942, comprises an envelope l0 enclosing a thermionic cathode H which is surrounded by a control grid i2 and a screen grid It to constitute the source ofoppositely directed electron beams which are directed by a curved electrode i5 along the inner surface of the electrode to a secondary electron emitter I6 positioned to intercept and to be bombarded by the electron beams from the thermionic cathode I I The secondary electrons from the emitter l6 pass through a grid-like output electrode IT to another secondary electron emitter l8, the secondary electrons from both emitters l6 and I 8 being collected by'the output electrode I1. tube of this type, in which the secondary electron emitters are subjected to rather severe usage, it has been found that secondary electron emitters l6 and I8 when made in accordance with my invention remain stable and eflicient over a wide range of temperature and withstand temperatures as high as 500 C. or 600 0. without loss of efliciency.

Fig. 2 shows diagrammatically and on a very enlarged scale a cross-section of an emitter constructed in accordance with my invention when embodied in an electrode having a Nichrome foundation. As appears from this figure the secondary electron emitting surface comprises a Nichrome foundation 20, a surface layer 2| presumably containing caesium and caesium oxide, and an intermediate film 22 formed in place on the base and containing chromic oxide, the film preferably being of such thickness that it shows By analogy With what is known of caesiated oxides of other metals it seems probable that the surface layer 2| 'has a caesium surface and comprises substratum in which oxides of caesium and of chromium are present, but the precise structure has not been determined. y

A convenient method of making a secondary electron emitter in accordance with my invention is to produce on a bright Nichrome electrode cleaned in chemically dry hydrogen a very thin film containing chromic oxideby heating the electrode to a bright red heat and degassing it while on the pump, reheating it in an atmosphere of oxygen at several millimeters pressure until interference colors appear on cooling, and pumping out the oxygen. activated by exposing it to vapor of caesium, rubidium or other activating metal in vacuum, preferably by flashing a caesium or rubidium pellet, until a metallic mirror appears in the bulb, which shows that activating metal has condensed on the electrode and on the bulb walls. The activating treatment is continued by baking the tube at about 250 C. for a short time to drive off the excess of activating metal from the electrode, admitting oxygen to a pressure of several millimeters of mercury for a few seconds while the electrode is heated by high frequency induction, pumping out the oxygen, again introducing into the tube the vapor of caesium or other activating metal, and again baking out the tube at about 250 C., which is a temperature high enough to cause a caesium absorber in the tube, such as stannic oxide, to take up excess caesium. A method of producing a thicker chromic oxide containing layer is to reheat the degassed Ni- Ina chrome electrode to moderate temperature while I on the pump, and expose it to moist hydrogen instead of to oxygen until the surface of the Nichrome electrode becomes greenish, which is the color of chromic oxide. Under these conditions the chromic oxide probably predominates in the film, as chromic oxide is not reduced, while the oxides of the metals of the base are either reduced or do not form freely. The electrode is then activated as above described.

I have produced by the methods above described secondary electron emitters which consistently give an emission ratio of secondary electrons to impact electrons of better than two to one at volts and of about four to one at 300 volts impact velocity. This secondary electron emitter is stable over a widerange o1 temperature, and the secondary electron emissivity is well maintained for a reasonable life up to at least a dull red heat, corresponding to a temperature of about 500 or 600 C., which is much above the temperature at which the caesium oxide on silver emitters becomes practically inoperative.

I claim:

1. An electrode comprising a foundation of-an alloy containing 10% to 20% chromium and the The electrode is then balance metal of the iron group, an oxidized alloy film of which a substantial part is chromic oxide on the surface of said foundation and of a thickness which produces interference colors, and a coating having a ratio of secondary electron emissivity greater than unity and comprising alkali metal condensed on the surface of said film.

2. A cathode consisting of a foundation of an alloy containing 10% to 20% chromium and the balance metal of the iron group, said foundation having on its surface an oxidized alloy film of which a substantial part is chromic oxide and activated by an alkali metal and an oxide of said alkali metal.

3. The method of producing a cathode of high secondary electron emissivity and stable over a temperature range up to red heat which comprises producing on the surface of a foundation said activating metal.

WARREN R, FERRIS. 

